Solid oxide fuel cells can have several advantages when they are operated at as high a temperature as possible. For example, the diversity in fuel use can be achieved, they can have MW-class capacity or more and thus be used in large-scale power plants, and high-temperature exhaust gas can be used to additionally generate power using a gas turbine. However, in the existing technology for manufacturing a solid oxide fuel cell, because of characteristics in which a unit cell is manufactured using a thin ceramic plate and a plastic process must be conducted at the final stage of the manufacturing process, it is difficult to significantly increase the area of the unit cell. Furthermore, there is no sealer that can be normally used at high temperature. In addition, because an electrical connector between unit cells made of ceramic is manufactured by stacking different kinds of metal plates one on top of another, problems of mechanical stress and high-temperature corrosion of metal are unavoidable. Given this, solid oxide fuel cells of only about 20 KW have been developed to date.
To avoid the use of such a metal electrical connector, a stack was proposed, which uses a segmented type cell integrated tube configured such that small unit cells are formed on a tubular support in the longitudinal direction and are electrically connected in series to each other by a conductive ceramic connector provided between the adjacent unit cells. This stack is advantageous in that the tubular support is made of inexpensive nonconductive ceramic rather than metal. However, because electric current flows in the longitudinal direction of an electrical connector, a conductive semiconductor and a cathode layer that have thin membrane structures, resistance is large, so that current density per unit area is low, and the conventional problems of electrical connection between the cell tubes still remain. Furthermore, it is impossible to three-dimensionally increase the size of the stack using small tubes. Moreover, the stack has no internal reformer so that if the size of the stack is increased, a problem of a temperature difference is caused, and it is not easy to control heat.
Solid oxide fuel cells (hereinafter referred to as SOFCs) are operated at high temperature ranging from 750° C. to 1,000° C. so that the efficiency is highest compared to those of other fuel cells. However, due to characteristics of a unit cell that must be formed of a thin ceramic plate and be processed through a plastic process, it is difficult to significantly increase the area of the unit cell. Furthermore, there is no sealer that can be normally operated at high temperature. In addition, between the unit cells made of ceramic, a gas channel and an electrical connection plate which functions to electrically connect the unit cells to each other are made of metal. Therefore, a problem of mechanical stress resulting from a hybrid stacking structure using metal and ceramic and a problem of high-temperature corrosion of metal are inevitable. Furthermore, planar unit cells are physically stacked one on top of another and are electrically connected only in series to each other. Thus, there is a problem in that if the performance of only one cell deteriorates, the performance of the entire stack also deteriorates. Hence, all the unit cells not only must be perfectly manufactured but the cell stacking structure for electrical connection must also be perfectly made and operated. However, the cell stack is sealed by a sealer, so even when only some of the cells malfunction, it is impossible to replace a portion of the stack with a new one or repair it. As such, although SOFCs can have several advantages when they are operated at as high a temperature as possible, for example, the diversity in fuel use can be achieved, they can have MW-class capacity or more and thus be used in large-scale power plants, and high-temperature exhaust gas can be used to additionally generate power using a gas turbine, the above-stated problems make it impossible to realistically make a unit cell having a large area of 400 cm2 or more or a large size of stack of 20 KW or more.
Zirconium oxide that yttria is added thereto and has had its crystal structure stabilized has been used as electrolyte material of unit cells of such a SOFC. Although this material has conductivity with oxygen ions, only when it is within a high-temperature range from 750° C. to 1,000° C. can the conductivity which is required to function as the fuel cell be provided. Given this, the operating temperature of the SOFC is typically 750° C. or more, and conductive material which has high-temperature resistance is used as the material of the electrodes, for example, a cathode into which air is drawn is made of LaSrMnO3, and an anode into which hydrogen is drawn is made of Ni—ZrO2 compound (cermet). In planar type SOFCs, a unit cell is configured in such a way that the anode or electrolyte support is coated with the other electrode or electrolyte layer to eventually form a unit electrolyte-electrode assembly (hereinafter referred to as an ‘EEA’) having a thickness of 1 mm or less, and then the EEA is provided with an electrical connection plate which has channels for supply of fuel gas or air to upper and lower layers of EEAs when they are stacked one on top of another, and is made of conductive metal to make it possible to electrically connect the opposite electrodes of the EEA to each other. Such a planar type SOFC has an advantage in that the EEA layer is thin. However, because of the characteristics of ceramic, it is difficult to control the degree of uniformity in the thickness or the planarity so that the size of the SOFC cannot be easily increased. Furthermore, when ceramic EEAs and metal electrical connection plates are alternately stacked one on top of another to form a unit cell stacking structure, a sealer is applied to the peripheries of the cells to realize a sealing structure for fuel gas and air between the cells. However, typically, the lowest softening temperature of glass-based material which is used as sealing material is about 600° C., but the SOFC must be operated at a higher temperature of 750° C. or more to obtain the satisfactory efficiency. Therefore, there is the possibility of leakage of gas resulting from the sealer softening. Moreover, because the stack is configured such that the unit cells are electrically connected only in series to each other, all the unit cells must be perfectly operated without any defect. The above-stated technical problems have made it difficult to commercialize the SOFC up to now.
To compensate for the disadvantages of the planar type cell, techniques that pertain to unit cells and stacks using flat tubular supports were proposed in U.S. Pat. Nos. 6,416,897 and 6,429,051. However, in these cases, additional gas channels for supply of gas to the cathode or anode and metal electrical connection plates must be provided on outer surfaces of flat tubular cells when a stacking structure is formed. This flat tubular structure can increase the mechanical strength of a cell stack, but because of characteristics such as the electrical connection plate being made of metal, when the cell stack is operated at high temperature, mechanical and thermal stress is caused between EEA layers made of ceramic. Moreover, the cells are connected only in series to each other, so a burden of zero-defect manufacture is still present.
In an effort to overcome the above-mentioned problems of the conventional SOFCs, a segmented type cell tube was proposed. The segmented type cell tube is configured in such a way that small units, each of which includes an anode, an electrolyte, and a cathode, are applied to the tube at positions spaced apart from each other in the longitudinal direction of the tube at regular intervals, and the cathode of each unit cell is electrically connected to the anode of the adjacent unit cell by an electrical connector. The segmented type cell tube is characterized in that depending on the number of cells connected in series to each other on the tube, the output voltage can be adjusted. Therefore, the output voltage of the segmented type cell tube can be greater than that of the typical flat tubular type, despite the fact that the segmented type cell tube does not have a physical stacking structure. Further, nonconductive material such as alumina which is inexpensive can be used as a support of the tube. However, because current flows in the longitudinal direction of the unit cells and electrical connectors that have thin membrane structures, resistance is increased so that power density is reduced. In addition, to form additional air or fuel gas channels, tubes must be arranged or stacked one on top of another at positions spaced apart from each other. In this case, it is difficult to electrically connect the tubes to each other.
In detail, in the segmented type fuel cell using the tubular support, if the unit cells are manufactured to form a cathode-supported type structure, air is supplied into the tube while fuel is supplied to space defined around the tube. Therefore, due to the reducing atmosphere around the tube, general metal material can be used as the material of the electrical connectors. The mechanical characteristics and high-temperature stability of the cathode-supported type tube are very superior, but electrical resistance in the cells is high so that output loss is increased. Thus, actual electrical output is typically 200 mW/cm2, in other words, comparatively low, and the production cost is high. On the other hand, in the case of an anode-supported type, fuel and air can be supplied in a manner opposite to that of the cathode-supported type. However, this case has a problem of corrosion of electrical connectors disposed outside the tube. In the case of the tubular type, it is also difficult to electrically connect tubes to each other. A method in which the tubular cell tubes are electrically connected to each other using wires or the like was proposed in US 2007/0148523 A1.
To mitigate the problems of such a tubular type structure, flat tube segmented type structures in which unit cells are arranged on a flat tube in the longitudinal direction in the same manner as that of the tubular type structure were proposed in U.S. Pat. No. 7,399,546 B2, JP 2006-172925 A, etc. A method of installing flat tube segmented type cell module tubes in a stacking manner is as follows. In the same manner as that of the typical flat tube type structure, cells may be provided on a cell or stack installation port using different kinds of components or the like and then it may be sealed by glass or the like (in JP 2003-282107 A), or alternatively, they may be directly connected to each other by brazing without using components (in JP 2006-172925 A). In such segmented type structures, although electrical connection between the segmented cells was illustrated as being realized by applying an internal connection layer made of LaCro3 or the like to the tube, there was no method of solving a problem of a reduction of the efficiency resulting from an increase in resistance that is caused in the structure in which the thin membrane is applied to the tube to form the electrical connector. Further, there was no method of providing electricity from the cell tube in which segmented unit cells are arranged or stacked. In the same manner as the typical flat tube type structure, even if an electrical connector is attached to one side of the cell module and the cell modules are connected in parallel or series to each other, when it is operated at a high temperature of 700° C. or more for a long time of period, deformation in the shape of the cells may reduce the contactability between the cell modules and metal electrical connectors. Moreover, problems of oxidation corrosion of the metal electrical connectors, an increased in resistance, etc. still remain.